Page 400 - Petrophysics 2E

368 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

with the surface, forming a preferentially oil-wet surface. Interfacially
active compounds-those that tend to accumulate at the interface-can
lower the interfacial tension and affect the wetting characteristics of the
fluid-rock system. Many of the surface properties of shales, sandstones,
and carbonates that affect the relative wetting of the surfaces by water
and crude oils are readily explained by examining the general chemical
structures associated with the principal minerals.
Yaalon determined the composition of 10,000 shales and arrived at
the following average composition: (1) 60% clay minerals-mostly illite,
(2) 20% quartz, (3) 10% feldspar, (4) 6% carbonates, (5) 3% iron oxide,
and (6) 1% organic matter [26]. The dominant characteristics of shales
are their ion exchange properties, electrical conductivity, swelling,
and dispersion when treated with fresh (or low-salinity) water. These
properties are attributed to the dominant presence of the minerals. Many
of these properties can be explained using an idealized, general structure
of clays (Figure 6.5). This is not an exact structure; clay structure
is three-dimensional and varies considerably from one type of clay to
another. The silicon atom is small and has a very strong charge of plus
four; therefore, in the case of silicon dioxide, the silicon atom's valence
is always satisfied with strongly bonded oxygen. As a consequence of the
strong silicon-oxygen bond, the clay mineral bonds are broken, leaving
oxygen exposed with its negative charge. This negative charge is satisfied
by association with positively charged cations, principally sodium and
calcium as illustrated in Figure 6.5.
Cation exchange occurs when di- or trivalent ions enter in a stream
of brine and displace the monovalent cations that are loosely associated
with the clay mineral. The ion with the greatest charge attaches more
strongly and cannot be displaced easily by monovalent ions. This can

Na+ Na+
Ca++ 0- 0- 0-
I I I
-0 - Si - 0 - Si - 0 - Si - 0-
I I I
0 0 0 Ca++
I
0 - Si - 0-
I
0 0 0- Ca++
I
Na+
I
Na+ 0- - Si - 0 - Si - 0-
I I
0- 0-
Na+ NaS
Figure 6.5. Idealized, planar illustration of clay structure and exchangeable cation
association with the clay.